Groundwater quality is just as crucial as its quantity when considering suitability for various uses. As such, ongoing qualitative monitoring of groundwater is vital. Classifying groundwater quality across regions for both drinking and agricultural purposes provides valuable guidance for land-use planners and aids in groundwater resource protection. This study utilized qualitative and quantitative groundwater data from the Mashhad Plain collected between 2007–2008 and 2011–2012. Spatial and temporal variations in groundwater quality parameters were analyzed using ordinary kriging and point co-kriging methods within a GIS framework. The results of a 20-year study on groundwater decline in the Mashhad Plain (1992–2012) indicate that the lowest average annual drop was 0.16 meters in the gold area east of Chenaran city, while the highest was 2.3 meters in Jimabad. Over time, groundwater quality in the Mashhad Plain deteriorated, with a notable decrease from the 2007–2008 to the 2011–2012 water years. Data also revealed excessively high TDS concentrations in certain areas, rendering groundwater unsuitable for drinking in those locations. According to the statistics and standards applied in this study, the groundwater quality map showed that only 12.5% of the total study area was classified as suitable for drinking-water supply in 2011–2012, compared with 18% in 2007–2008. For agricultural use, approximately 70% of the total study area was classified as favorable to moderately suitable in 2011–2012, compared with 77% in 2007–2008, whereas 30% was classified as unfavorable. Some areas were also identified as being at risk of salinization. In these areas, agricultural management measures, including the cultivation of salt-tolerant crops and provision of adequate drainage, are recommended.